Piezoelectric devices have long been known which convert electrical signals into motion. In particular, loudspeakers constructed of a flat plate and driven by piezoelectric wafers mounted on the plate are well known in the art as exemplified by the device disclosed in Kompanek U.S. Pat. No. 3,423,543. This type of device suffers from a major shortcoming in that adequate low frequency response cannot be achieved without using multiple drivers and/or a large radiating surface area.
Spitzer et al U.S. Pat. No. 2,911,484 discloses an electro-acoustic transducer in FIGS. 3 and 4 which has a diaphragm, a housing to which the diaphragm is mounted, a plurality of curved piezoelectric wafers mounted so that their peripheral edge is cemented to the underside of the diaphragm and their central portions are cemented to a boss on the housing. Spitzer et al makes it clear in their specification that a plurality of driving elements is necessary to achieve low frequency response. No teaching is found in Spitzer et al relating to the optimal size for the boss.
U.S. Pat. No. 3,721,840, Yamada, discloses a sound generator having a relatively thin diaphragm and a piezoelectric monomorph disc adhered to a metallic substrate which is adhered to the diaphragm over an aperture therein.